Implantable biological tissues can be formed of human tissues preserved by freezing (i.e., cryopreserving) the so called homograft tissues, or of animal tissues preserved by chemically fixing (i.e., tanning) the so called bioprosthesis (Carpentier, Biological Tissues in Heart Valve Replacement, Butterworth (1972), Ionescu editor). The type of biological tissues used as bioprostheses include cardiac valves, blood vessels, skin, dura mater, pericardium, small intestinal submucosa (“SIS tissue”), ligaments and tendons. These biological tissues typically contain connective tissue proteins (i.e., collagen and elastin) that act as the supportive framework of the tissue. The pliability or rigidity of each biological tissue is largely determined by the relative amounts of collagen and elastin present within the tissue and/or by the physical structure and configuration of its connective tissue framework. Collagen is the most abundant connective tissue protein present in most tissues. Each collagen molecule is made up of three (3) polypeptide chains intertwined in a coiled helical configuration.
The techniques used for chemical fixation of biological tissues typically involve the exposure of the biological tissue to one or more chemical fixatives (i.e., tanning agents) that form cross-linkages between the polypeptide chains within a given collagen molecule (i.e., intramolecular crosslinkages), or between adjacent collagen molecules (i.e., intermolecular crosslinkages).
Examples of chemical fixative agents that have been utilized to crosslink collagenous biological tissues include: formaldehyde, glutaraldehyde, dialdehyde starch, hexamethylene diisocyanate and certain polyepoxy compounds. Of the various chemical fixatives available, glutaraldehyde has been the most widely used since the discovery of its antiimmunological and antidegenerative effects by Dr. Carpentier in 1968. See Carpentier, A., J. Thorac. Cardiovascular Surgery, 58: 467-69 (1969). In addition, glutaraldehyde is one of the most efficient sterilization agents. Glutaraldehyde is used as the fixative and the sterilant for many commercially available bioprosthetic products, such as porcine bioprosthetic heart valves (e.g., the Carpentier-Edwards® stented porcine Bioprosthesis), bovine pericardial heart valves (e.g., Carpentier-Edwards® Pericardial Bioprosthesis) and stentless porcine aortic valves (e.g., Edwards PRIMA Plus® Stentless Aortic Bioprosthesis), all manufactured and sold by Edwards Lifesciences LLC, Irvine, Calif.
One problem associated with the implantation of many bioprosthetic materials is that the connective tissue proteins (i.e., collagen and elastin) within these materials can become calcified following implantation within the body. Such calcification can result in undesirable stiffening or degradation of the bioprosthesis. Two (2) types of calcification—intrinsic and extrinsic—are known to occur in fixed collagenous bioprostheses. Intrinsic calcification follows the adsorption by the tissue of lipoproteins and calcium binding proteins. Extrinsic calcification follows the adhesion of cells (e.g., platelets) to the bioprosthesis and leads to the development of calcium phosphate containing surface plaques on the bioprosthesis.
The factors that affect the rate at which fixed tissue bioprostheses undergo calcification have not been fully elucidated. However, factors thought to influence the rate of calcification include the patient's age, the existence of metabolic ‘disorders (i.e., hypercalcemia, diabetes, etc.), dietary factors, the presence of infection, parenteral calcium administration, dehydration, in situ distortion of the bioprosthesis (e.g., mechanical stress), inadequate anticoagulation therapy during the initial period following surgical implantation and immunologic host-tissue responses.
Various techniques have heretofore been proposed for mitigating the in situ calcification of glutaraldehyde-fixed bioprostheses or for otherwise improving the glutaraldehyde fixation process. Included among these are the methods described in U.S. Pat. No. 4,729,139 (Nashef) entitled Selective Incorporation of a Polymer into Implantable Biological Tissue to Inhibit Calcification; U.S. Pat. No. 4,885,005 (Nashef et al.) entitled Surfactant Treatment of Implantable Biological Tissue To Inhibit Calcification; U.S. Pat. No. 4,648,881 (Carpentier et al.) entitled Implantable Biological Tissue and Process For Preparation Thereof; U.S. Pat. No. 4,976,733 (Girardot) entitled Prevention of Prosthesis Calcification; U.S. Pat. No. 4,120,649 (Schechter) entitled Transplants; U.S. Pat. No. 5,002,566 (Carpentier) entitled Calcification Mitigation of Bioprosthetic Implants; EP 103947A2 (Pollock et al.) entitled Method For Inhibiting Mineralization of Natural Tissue During Implantation, and U.S. Pat. No. 5,215,541 (Nashef et al.) entitled Surfactant Treatment of Implantable Biological Tissue to Inhibit Calcification. Recently a new technique of calcium mitigation by high temperature fixation of the tissue in glutaraldehyde has been developed and was described in U.S. Pat. No. 5,931,969 (Carpentier et al.) entitled Methods And Apparatus For Treating Biological Tissue To Mitigate Calcification. Although some of these techniques have proven to be efficient in reducing calcification, there remains a need in the art for further improvements of the existing techniques or for the development of new calcification-mitigating techniques to lessen the propensity for post-implantation calcification of fixed bioprosthetic tissues.